The most successful type of thermionic converter to date is the elementary vapor diode converter, which includes two electrodes, the emitter and the collector. The interelectrode space is filled with a vapor such as cesium vapor. A low voltage arc is generated between the two electrodes to provide positive ions necessary to neutralize the space charge effect caused by the electrons moving from the emitter to collector which degrades converter performance. The potential difference across the interelectrode space required to maintain this arc, known as the arc drop, typically, is at least .4 volts. Since this potential difference is directly subtracted from the output voltage of the converter, it greatly reduces the electrical power output. In addition, for the electrodes to generate ions and for the emitter to emit electrons when heated, requires that operating temperatures of the emitter be very high.
Various devices have been attempted to supply sustaining ions by auxiliary means to eliminate the arc drop and allow for operation at low emitter temperatures. For example, a plasmatron employs a third electrode from which an auxiliary arc, maintained by an external power supply, is used to generate the ions for sustaining the main discharge current between the electrodes. A plasmatron suffers from great inefficiency because the net power generated by the converter is reduced by the power requirements of the external power supply, and suffers from being uneconomical because of its complexity due to the need for heating and electrically insulating the third electrode and providing the third electrode with an independent vacuum envelope feedthrough. In addition, plasmatrons require the auxiliary emitter to be placed in the main electrode gap in order to achieve efficient transport of ions into the main electrode gap. Since it is advantageous to operate with small main interelectrode spacing, this requirement makes it difficult to approach optimum operating conditions.
It is therefore an object of this invention to provide an improved gas-filled thermionic converter.
Another object of this invention is to provide an improved means of supplying sustaining ions for the main discharge region of a gas-filled thermionic converter.
Another object of this invention is to provide an improved gas-filled thermionic converter of simple construction and of low operating temperature.